1. Field of the Invention
The present invention relates to variable capacitance devices including a MEMS structure, and specifically to variable capacitance devices that include a control circuit that electrically controls a distance between opposing electrodes.
2. Description of the Related Art
Currently, many attempts are being made to apply variable capacitance devices that use a MEMS structure to, for example, matching circuits in the front end portions of wireless communication high-frequency modules for cellular phones.
A variable capacitance device that uses a MEMS structure includes a mechanical unit that actually functions as a variable capacitance device and a driving circuit that applies a driving voltage to the mechanical unit, as described in Japanese Unexamined Patent Application Publication No. 2008-181725 and Japanese Unexamined Patent Application Publication No. 2009-70940.
The mechanical unit includes a fixed flat plate and a movable flat plate made of, for example, silicon. One end or both ends of the movable plate are supported by fixed members, thereby forming a cantilever or doubly supported beam, and as a result of this structure, the movable plate is arranged so as to be spaced apart from the fixed plate by a predetermined distance and so as to be partially movable. A pair of driving electrodes and a pair of opposing electrodes which function as a variable capacitor are formed so as to respectively face each other on the fixed plate and the movable plate in a range in which the movable plate moves.
When a predetermined driving voltage is applied to the pair of driving electrodes by the driving circuit, the driving electrodes are attracted to each other by an electrostatic attraction, whereby the movable plate deforms. As a result, the distance between the driving electrodes is decreased and the distance between the opposing electrodes for variable capacitance is also decreased accordingly. Hence, the capacitance of the variable capacitance device becomes higher than when the driving voltage was not being applied. On the other hand, when application of the driving voltage is stopped, the electrostatic attraction between the driving electrodes is lost, and the distance between the opposing driving electrodes for variable capacitance returns to the original state prior to the application of the driving voltage because of a restoring force of the movable portion. As a result, the capacitance of the variable capacitance device becomes lower than when the driving voltage was being applied.
When the variable capacitance device is used in a matching circuit for high-frequency communication, as described above, it is preferable to realize as many capacitance values as possible in order to widen the range of matching and increase the capacitance resolution for matching.
Here, when three or more capacitance values are to be obtained, it is necessary to use a configuration in which a plurality of variable capacitance devices with a structure providing two capacitance values are formed and connected in parallel or to make a single variable capacitance device have a structure that provides three or more capacitance values.
A variable capacitance device providing two capacitance values can be realized using a simple structure and control in which opposing driving electrodes for variable capacitance are made to contact each other with, for example, a dielectric layer therebetween through application of a driving voltage, and the opposing driving electrodes for variable capacitance are separated from each other by stopping the application of a driving voltage.
However, when multiple values of capacitance are realized using such variable capacitance devices providing only two capacitance values, many variable capacitance devices need to be formed and, in addition, circuit patterns for connecting these devices need to be formed. As a result, reductions in size and space are difficult. Further, the amount of signal loss due to the connection patterns cannot be ignored.
On the other hand, when multiple values of capacitance are realized using a single capacitance device, reductions in size and space can be realized. To put it simply, adjustment of a driving voltage allows adjustment of the distance between opposing electrodes for variable capacitance, i.e., the distance between driving electrodes, in accordance with capacitance, whereby multiple values of capacitance are realized. In addition, since the circuit patterns for connecting a plurality of capacitors in parallel are not required, signal loss can be minimized.
However, with existing configurations, it is actually not easy to control the driving voltage so as to maintain the capacitance at a constant value because of, for example, the aging deterioration of a movable plate, self actuation, variations in the driving voltage, and instantaneous deformation of the movable plate due to external factors.